I have a growing number of PCs and laptops in my home that need a backup solution. My question is 2 fold:

Most commercial backup vendors
appear to license by the machine,
are there any products that give a
license that can be used on all your
machines, without a per machine
cost?

An alternative is to have a
centrally controlled back up
strategy controlled from a single PC
with an attached hard drive that can
back up other machines on the
network. What backup software is
suitable for this approach? Will
this software be windows/Linux
interoperable?

I've searched for duplicate questions but don't see anything that addresses the multi machine/cost issue.

OSs in network will be Windows (XP, Vista, 7) and Linux variants.
Not pattern to when machines will be switched on.

A lot will depend on the size and spread of data you wish to backup on the machines, and how they will generally be connected (do you want to update the backup of a laptop when you have it off-site, for instance).

I use a technique similar to those described in this very useful article to maintain offsite backups of my docs and settings from my main box, netbook, and mail server - it should be quite easy to implement on a number of machine if the files you backup are in consistent places (just one script to copy to all the machines, and setup a SSH key pair for each for authentication).

The rsync algotithm is a very efficient way to maintain backups of potentially large amounts of data even over slow links, and being a command line utility it is easily used in scripts that you can run manually or automate via cron or windows' scheduled tasks. rdiff is another similar option, which uses the rsync algorithm to create "delta" files for each run (rdiff-backup wraps this utility to produce an arrangement that works much the same as rsync+hardlink snapshots). There are a number of extra front-ends available for them too, which either just add a GUI front-end, wrap them to provide a specific arrangement in a more friendly way (though losing some flexibility), or both.

There are a number of ports of rsync+ssh for Windows (the one that is part of cygwin being the one most people use, cwRsync is another apparently popular option). Though you might need to be running Linux/BSD server-side to use the hardlink-based snapshot technique (I'm not sure if the Windows ports support hard links though they may as the NTFS filesytem can support/emulate them), if you are sticking using Windows server-side rsync will otherwise work well and rdiff's technique doesn't need hardlinks anyway.

A solution that I've used successfully in the past is BackupPC. It runs on a single server and periodically pulls backups over rsync or ssh or samba from all the other machines. It has many useful features, too many to describe here.

I can heartily second this recommendation. I have it backing up a mix of Windows and Linux boxes. You mention: "Not pattern to when machines will be switched on." -- BackupPC is especially good here, in that it has the ability to poll for hosts periodically and then give priority to hosts that have been "missing" the longest.
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Joe CasadonteJan 30 '10 at 12:20

This won't handle backup for your Linux machines but for the Windows machines on your network, I highly recommend Windows Home Server. The license cost is managable ($100) and it'll handle up to 10 machines, which is sufficient for home use.

An option for the linux hosts would be to rsync criticial data to a fileshare on one of the windows machines and make sure that that gets backed up regularly via the WHS.

Syncplicity allow you to use "as many computers as you like" with the standard subscription. If you don't want to take advantage of their synchronization functionality, you should be able to turn that off, and just backup all your machines using the same account/license on each.